It is known in the art to create a heterojunction acoustic charge transport device (HACT) having a plurality of layers typically comprising a charge transport layer surrounded on its upper and lower surfaces by charge confinement layers. Above the upper charge confinement layer (at the external air interface), is typically a cap layer. All of the aforementioned layers may be grown above a substrate, such as gallium arsenide (GaAs). The lower charge confinement layer is typically made of undoped (or not intentionally doped) aluminum gallium arsenide (AlGaAs), and the upper charge confinement layer is typically made of N-doped AlGaAs (i.e., AlGaAs doped with an N-type dopant). The charge transport layer is typically made of undoped GaAs. However, other semiconductors having piezoelectric properties known to those skilled in the art may also be substituted for these materials. It is also known that a surface acoustic wave (SAW) may be launched into the HACT structure by known means, such as an interdigital SAW transducer. Further, charge may be injected into the structure at one end and be carried by the SAW (in groups called "charge packets") along the charge transport layer to another where it is removed. The charge carried by the SAW stays confined to the charge transport layer because the charge transport layer material has a conduction band energy lower than that of the surrounding charge confinement layers. Such a HACT device is described in commonly-owned U.S. Pat. No. 4,893,161 to Tanski et al, which is incorporated herein by reference.
It is also known in the art that HACT epitaxial layer structures have "surface states" at the air/cap layer interface (i.e., the external surface of the cap layer). Surface states are a well known phenomena which exhibit trapping and recombination sites for mobile charge carriers. It is speculated by those skilled in the art that surface states are created due to imperfections (defects) in the crystalline structure at the external surface of the device which cause loose molecular bonds. However, it is known that surface states "trap" (attract and hold) electrons from, or supply electrons to "recombine" with, the charge packets propagating within the charge transport layer, thereby distorting the information carried thereby.
Prior attempts to reduce the effects of the surface states have included an N-doped GaAs cap layer, whereby the dopant electrons are intended to fill the surface states so that electrons transported by the SAW do not get trapped by the surface states. The precise doping concentration for satisfying surface state traps depends on the number of traps at the surface, which can vary depending upon material processing. However, even if all the traps are satisfied by donor electrons, the surface states will still cause carrier recombinations because the bonds to the surface states are not strong. Similarly, running an initial group of charge packets through the system at power-up in an attempt to fill the surface states suffers the same results (i.e., electrons would be attracted to the surface states and subsequently leave the surface states and recombine with the charge packets).
Therefore, it is desirable to reduce the effects of surface states in a predictable and reproducible manner in order to improve the charge transport efficiency along the charge transport layer.